It is often desirable to evaluate the properties of a material without damaging the specimen being tested. Several known techniques make use of magnetic measurements to evaluate specific properties in a specimen sample. Advances in those techniques provide the ability to obtain meaningful information regarding less pronounced intrinsic properties of a material through measurement of magnetic properties. For example, it is known that coercivity can be used to detect plastic deformation and hardness, that maximum differential permeability can be used to measure stress, that remanence can be used to detect creep damage, that a combination of remanence and coercivity can be used to detect impending fatigue failure, and that hysteresis loss can be used to detect changes in grain boundary segregation arising from temper embrittlement.
This ability to correlate the magnetic characteristics of a material to its less pronounced intrinsic physical properties is disclosed in U.S. Pat. Nos. 5,008,621 and 5,012,189, both of which issued to the present inventor. As is recognized in the art and taught by these patents, bulk magnetic properties such as coercivity, remanence, hysteresis loss, initial permeability, maximum differential permeability and anhysteretic permeability may be derived from magnetic hysteresis curves. As is well known, the magnetic hysteresis curve is a plot of flux density B in a material with respect to a varying applied magnetic field intensity H. Both the '621 patent and the '189 patent further teach that information regarding the physical properties of a sample specimen may be obtained from the evaluation of the magnetic properties occurring therein.
In order to obtain the hysteresis curves important to such studies, a magnetic sensing head is required to magnetically excite the material and measure its response. Typically, magnetic inspection heads have consisted of a substantially "C" shaped core having a pair of legs terminating in a pair of co-planar pole faces, so that when the core is placed on a planar sample, both pole faces are in contact with the surface of the sample. The core is wrapped with a power coil which generates a magnetic field when energized. A sensing coil is also wrapped on the core to detect the magnetic flux .PHI. in of the material under the action of the applied field. The core is placed on the surface of a sample such that a magnetic circuit is created including the core and the test material. When both the surface of the test material and the pole faces of the inspection head are planar, placement of the sensing head on the surface of the sample creates a magnetic circuit which has only very small air gaps between the pole faces and the sample. If the sample is not planar, larger and often variable air gaps will result, with the variation depending on how the inspection head is placed on the sample. The air gaps will have a substantial effect on the measurements being taken, and thus the system is no longer measuring primarily the characteristics of the material, but also the characteristics of the variable air gap. As a result, the ability to analyze materials which are non-uniform or non-planar has encountered these problems of the variable air gap.
Thus, while magnetic measurement techniques have progressed using automated and computerized systems as disclosed in the above-referenced patents, difficulties have been encountered in taking magnetic measurements of non-planar surfaces. It is well appreciated that a magnetic inspection head is not a point contact device, and must have two areas of preferably uniform contact with the surface of the specimen to be tested. The magnetic field is coupled into the sample via those two areas, and the sample in effect becomes part of the magnetic circuit together with the inspection head. When the surface is non-planar, undulating or contoured, it is difficult if not impossible to get uniform contact between the inspection head and the surface using the standard C core inspection head. The repeatability of the measurements can therefore be very dependent on the technique of the user in applying the inspection head to the surface. For example, if the inspection head has two co-planar poles which are on the order of 1.0 square inches in area separated by a gap of about 1 inch, if the surface to be measured does not provide a planar area capable of accommodating that probe, the probe will then be applied with at least some area of the pole faces having a substantial air gap separating it from the sample. The manner in which the operator applies the inspection head to the sample will determine how much of the pole face is in contact with the surface and how much is separated by an air gap of unknown dimension. The repeatability of such measurements from sample to sample, or the ability to measure a sample and calibrate it to a standard in such an arrangement is very severely limited.
Even the ability to take magnetic measurements on non-planar surfaces of substantially uniform contour, such as large pipes, can be adversely impacted by the use of typical prior art probes. For example, if one limits the measurements to the axial direction of the pipe, and tries to arrange the probe such that the poles are tangential to the surface, the measurements may be repeatable from measurement to measurement. However, whenever the probe is displaced slightly from its intended position, the readings will be impacted, even in this not usual measurement situation.